A typical grindstone of an electric cutting-tool grinder has a disk shape. The circumferential surface or the end surface of the grindstone is used as a grinding surface. Since the thickness of the grindstone is relatively small, the width of the grinding surface is small when the circumferential surface is used as the grinding surface. Thus, it is not suitable for cutting-tools with a long blade such as a kitchen knife. When the end surface of the grindstone is used as the grinding surface, the blade of the kitchen knife is abutted against the circular grinding surface to cross the grinding surface. Thus, two different grinding motions are simultaneously performed in which the grinding surface rotates toward the blade and in which the grinding surface rotates away from the blade. As a result, half of the blade is pressed by the grinding surface, and the remaining half of the blade is pulled by the grinding surface. Thus, the force that presses the blade and the force that pulls the blade apply force to the blade in the same rotation direction. This tends to rotate the kitchen knife. Thus, the kitchen knife will be unstable and the grinding operation will be difficult to perform.
To resolve such a drawback of the disk-like grindstone, a grindstone has been disclosed in Patent Document 1 that has a wider grinding surface and efficiently grinds a blade. According to this grindstone, a conic surface is formed on the circumferential surface of a disk, and the conic surface is used as the grinding surface. Thus, the width of the grinding surface is increased. Since the grinding surface rotates in one direction with respect to the blade by abutting the blade along the generating line of the conic surface, the entire blade is ground uniformly. Also, since grinding is performed in the range of the length of the generating line, the grindstone is capable of grinding the kitchen knife with a long blade.
However, the grinder of Patent Document 1 has a great drawback with regard to a kitchen knife that needs to be ground on both surfaces of the blade, that is, first and second blade surfaces such as those of an usuba hocho (Japanese vegetable knife). There is no problem when grinding the first blade surface of the usuba hocho, but there is a problem when grinding the second blade surface. More specifically, when grinding the first blade surface of a thin-bladed kitchen knife, an operator first grasps the handle of the kitchen knife with the right hand, and abuts the first blade surface against the generating line of the grinding surface formed by the conic surface. At this time, the grindstone rotates in a direction away from the blade, and grinds the kitchen knife without any problem.
The generating line of the grinding surface formed by the conic surface corresponds to a contour line on both sides when a truncated cone-shaped grindstone is viewed from the front.
When grinding the second blade surface, if the kitchen knife held in the right hand is turned over and ground while the kitchen knife is held in the right hand, the rotation direction of the grindstone with respect to the blade is reversed. As described above, there is no problem in grinding when grinding the first blade surface since the grindstone rotates in the direction away from the blade surface. However, in the case in which the second blade surface is ground while the kitchen knife that has been turned over is held in the right hand, the grindstone rotates toward the blade surface. Thus, the grinding surface collides against the blade edge, and the kitchen knife might be lifted while grinding. Therefore, it is difficult to grind smoothly.
Thus, the distal end of the blade and the handle of the kitchen knife are flipped over such that the handle of the kitchen knife is located on the left side of the operator's body by switching the kitchen knife to the left hand. When the kitchen knife is turned over in this manner and the second blade surface is ground, the grindstone rotates in the direction away from the second blade surface like the first blade surface. However, if the handle of the kitchen knife is located on the left side of the operator's body, when moving the kitchen knife during grinding, the hand and the handle of the kitchen knife interfere with the frame of the grinder, and the movement of the kitchen knife is hindered. Thus, the grinding operation cannot be substantially performed.
Also, when grinding the kitchen knife held in the right hand, it is preferable to place the left hand on the blade so that the kitchen knife will be stable. However, in the case with the grinder of Patent Document 1, the frame gets in the way and the operator cannot place the left hand on the blade.
Furthermore, in the grinder of Patent Document 1, the motor is exposed and noise during use is loud. Such a grinder is not suitable for household use also in the aspect of the design. Thus, in order to sell it to the general public, the motor is preferably covered by a housing. Furthermore, if a grindstone that needs grinding water is used in this grinder, the grinding water falls onto the motor. Thus, when mounting the grindstone that needs grinding water, the motor is also preferably covered by the housing.